RE: Shotgun barrel length?
Sometimes the length of the barrel determins how well your shot will pattern. Not really the size of the pattern, but how well the shot is distributed thru out the pattern. In most cases a longer barrel will pattern more evenly from what I have seen.
I have never hunted water fowl, but I do turkey hunt. You don't just buy some shells and head out. I mean you can, but I wouldn't suggest it. Just like a rifle you have to buy some different kinds of ammo to see what works best in your particular weapon. I don't just look for a specific size of pattern, but more one that is very even thru out the pattern. When shooting at a target the size of a turkeys head you don't want big holes or gaps in the pattern. The only way to know is to shoot different brands and shot sizes to see what works best out of your barrel and choke.
Normally a longer barrel or more so a longer choke will not distort or deform the pellets as they go thru the choke. I think it actually has more to do with the length of the choke than the barrel to be honest.
And like said above a longer barrel will point and track better in most cases. At least that is what I've been told from friends that shoot skeet and trap.
Good luck,
Paul